These days, the likelihood of fault current in domestic power systems is gradually increasing due to continuously increasing power loads. Additionally, for domestic systems, land is relatively scarce, and thus system lines are relatively short, and the systems are used in the state of being closely connected with each other for easy maintenance and repair, undesirably further increasing the risk of fault current.
Therefore, some breakers preinstalled in domestic transmission lines have to be replaced with new ones, or the lines have to be separated. With the goal of solving these problems, a current limiter is proposed, which is a novel power device for reducing or limiting the fault current of power systems to a level equal to or less than the breaking capacity of preinstalled breakers.
In power systems, a current limiter functions to limit mechanical, thermal and electrical stress, which are applied to bus bars, insulators, breakers and the like when fault current occurs due to system accidents. Because of a continuous increase in the risk of fault current in such systems and a difficulty in developing power devices for coping therewith, the demand for current-limiting elements with which fault current may be controlled is drastically increasing.
Although the development of current limiting techniques that are available in the real world is delayed, attributable to technical difficulties and difficulties in commercialization, high-temperature superconductors are newly discovered, and thus the likelihood of the development of current limiters using the non-linear voltage-current characteristics thereof has come to the fore. The development of high-temperature superconducting current limiters using liquid nitrogen as a refrigerant began in earnest in 1987.
A recent advance in superconducting elements for use in superconducting current limiters takes the form of a resistive superconducting current limiter using superconducting windings. Specifically, the magnet windings are formed into modules, which are then connected in series or in parallel. Depending on the type of winding process, the modules are broadly classified into solenoid modules and pancake modules.
The resistive current limiter and the current limiting module are required to be cooled because of the particularities of the superconductors. That is, the superconductor properties are not lost under the condition that the temperature is equal to or lower than a predetermined level. As for a superconductor used for the current limiter, a very large current flows in the event of fault current.
Such current is referred to quench current, which is equal to or higher than critical current. Resistance upon normal operation is almost ignored, and then instantly great resistance appears, thus causing joule heating. The joule heat thus generated is emitted by the peripheral cooling medium, whereby the superconducting current-limiting element may stably operate without electrical/thermal changes.
In a double pancake configuration for conventional superconducting magnets and current limiter modules, modules, in which an insulator is interposed between superconducting wires wound in pancake form, are connected in series to generate large flux density.
Furthermore, current is allowed to flow in the same direction, and thus a large external mechanical force is always applied to superconducting winding modules, undesirably causing instability thereof.
There is no cooling channel for effectively preventing the local generation of joule heat during operation, and heat is dissipated from the upper and lower surfaces of the windings by the refrigerant, and the inside thereof is cooled through conduction.
As described above, however, the superconducting winding modules in pancake form are manufactured using a stacking process so as to connect the ends thereof and to allow current to flow in the same direction.
The magnetic fields of individual modules generated by the current in the same direction overlap each other, whereby a magnetic field is formed in a predetermined direction in the inside thereof, and individual modules are bonded in different directions for non-inductive windings, thereby allowing the current to flow in opposite directions.
The magnetic fields having the same magnitude generated in a single module flow in opposite directions, and are thus offset, and the inductive component may be decreased, and thus internal flux may disappear, resulting in non-inductive windings.
In the double pancake configuration for superconducting magnets and current limiter modules, the pancake windings are stacked, and the inside of the windings is designed to radiate heat through conduction without the cooling channel, and only the two sharp tips of the superconducting tape are exposed to the refrigerant, so that heat is radiated to the refrigerant.
Thereby, the double pancake configuration for superconducting magnets and current limiter modules is problematic because the thermal energy generated by fault current is not efficiently emitted, and recovery characteristics for normal operation may be deteriorated.
In order to ensure stability against mechanical impacts upon normal operation, a material such as epoxy, which has low thermal conductivity, has to be incorporated into the surface thereof, which may make the emission of thermal energy more difficult.
In this regard, Korean Patent Application Publication No. 2007-0068929 (Current-limiting module for superconducting current limiter, laid-open date: Jul. 2, 2007) is disclosed.